communicable by infection, as from one person to another or from one part of the body to another: infectious diseases.
causing or communicating infection.
tending to spread from one to another: infectious laughter.
Law. capable of contaminating with illegality; exposing to seizure or forfeiture.
Obsolete. diseased.

Origin of infectious

First recorded in 1535–45; infect(ion) + -ious
Related formsin·fec·tious·ly, adverbin·fec·tious·ness, nounnon·in·fec·tious, adjectivenon·in·fec·tious·ly, adverbnon·in·fec·tious·ness, nounun·in·fec·tious, adjectiveun·in·fec·tious·ly, adverbun·in·fec·tious·ness, noun
Can be confusedcontagious infectious (see synonym study at contagious)

Synonyms for infectious

Synonym study

1. See contagious. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for infectious

Contemporary Examples of infectious

Historical Examples of infectious

  • "Evidently, of an infectious fever as your Eminence so well says," he replied.

  • As the doctor declared it not infectious, I was allowed to nurse him.

    Wilfrid Cumbermede

    George MacDonald

  • The laugh was so infectious that even Brown was obliged to smile.

    The Woman-Haters

    Joseph C. Lincoln

  • It automatically cleanses the wound of any infectious matter.


    Eliot H. Robinson

  • Her happiness was infectious, and Sarah turned to her visitor with an amused smile.

British Dictionary definitions for infectious



(of a disease) capable of being transmittedCompare contagious
(of a disease) caused by microorganisms, such as bacteria, viruses, or protozoa
causing or transmitting infection
tending or apt to spread, as from one person to anotherinfectious mirth
international law
  1. tainting or capable of tainting with illegality
  2. rendering liable to seizure or forfeiture
Derived Formsinfectiously, adverbinfectiousness, noun
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for infectious

"catching, having the quality of spreading from person to person," 1540s of diseases, 1610s of emotions, actions, etc.; see infect + -ous.

Online Etymology Dictionary, © 2010 Douglas Harper

infectious in Medicine




Capable of causing infection.
Caused by or capable of being transmitted by infection.
Caused by a pathogenic microorganism or agent.
Related formsin•fectious•ness n.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

infectious in Science



Capable of causing infection. See Note at contagious.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.